FAR FROM THE TOUR-BUS CROWD

Track gorillas in Uganda, explore a rain forest, or pedal across France

The Baliem Valley in the Indonesian province of Irian Jaya is about as far as a tourist can get from the modern world. But in May, an intrepid group of travelers over age 50, led by Toronto-based ElderTreks, took a jeep to the end of the valley's road. Then they hiked some 30 miles through mountainous terrain inhabited by Stone Age Dani tribes. Led by a native, English-speaking guide, they met the tribespeople, slept in thatched huts, and saw an eerie tribal funeral. ''It was fabulous,'' says Nadine Heyman, 65, a retired principal from Long Island in New York. ''I'd go back today.''

Forget sedate bus tours and restful cruises. Today, a new generation of retirees is ''hungry to go off the beaten path,'' says Alan Lewis, CEO of Grand Circle Travel in Boston, a specialist in senior adventure travel. Lewis says this $500 million segment of the travel business is growing 30% a year, driven in part by more and more retirees with time, money, and a yen for the exotic. Wildland Adventures in Seattle says seniors make up 60% of the participants in its ecotourism jaunts to such locales as the Costa Rican rain forest.

Senior adventure travel is booming because retirees are in better physical shape, more affluent, and more daring than in the past. Gene Wellman, 71, a retired environmental consultant from Klamath Falls, Ore., typifies the breed. A seasoned business traveler, Wellman has no desire to be herded onto sightseeing buses. So he and his wife, Genevieve, have joined small-group trips to French Polynesia and Peru.

As its name implies, adventure travel can be taxing and even risky. Abercrombie & Kent in Oak Brook, Ill., warns clients of the ''strenuous'' hiking and ''rustic'' conditions on its five-day gorilla tracking trip in Uganda. On such trips, decent medical care can be many miles away, so it's crucial to be realistic about your abilities and to screen tour operators. ''Ask about their insurance, the age range of participants, and how long they've been in business,'' as well as about emergency backup plans, says Jerry Mallett, president of the Adventure Travel Society in Englewood, Colo. And don't leave home without a medical checkup and insurance that allows for evacuation in case of a medical emergency. Many operators offer such coverage, but you can buy it on your own.

EASIER TERRAIN. Not every exotic tour is all that arduous, however. Grand Circle's Overseas Adventure Travel (OAT) unit offers a 17-day trekking trip in Nepal, but the hikes take place in the Himalayan foothills, where the elevation never tops 5,400 feet--about the same as Denver. Most of Abercrombie & Kent's African safaris are billed as ''easy'' or ''moderate.''

Exposure to exotic cultures, rather than physical challenge, is the main aim of many senior adventurers. On OAT's Sahara odyssey in Morocco, they visit an encampment of Berber nomads. And Radisson Seven Seas Cruises offers a 25-night voyage through the Northwest Passage in which travelers visit Inuit settlements and sail among icebergs.

All this fun doesn't come cheap. Costs range from $3,390 a person for OAT's Nepal trip to $15,000 and up for the Radisson cruise. At those prices, travelers expect--and get--a certain degree of pampering. Butterfield & Robinson's hiking and biking trips through France and Italy, popular with the 50-plus crowd, use hotels of the ultraluxe Relais & Chateaux chain. A far cry from roughing it, but such touches make these trips all the more appealing to well-heeled retirees--even those in hiking boots.